1. Field of the Invention
The present invention relates to the technique for growing single crystal of semiconductor such as silicon (Si) using the well-known Czochralski growth method and more particularly, to an apparatus for and a method of growing a single crystal of semiconductor, in which magnetic field is applied to a melt of semiconductor in a rotating crucible while an electric current is supplied to the melt so as to intersect with the magnetic field, thereby growing a single crystal of semiconductor from its seed crystal.
2. Description of the Related Art
Single-crystal semiconductor wafers, which have been used as substrates of ultralarge-scale integrated electronic devices (ULSIs), are produced from an ingot of a single crystal of semiconductor (e.g., Si). An ingot of a single crystal of semiconductor is typically obtained by crystal growth from a semiconductor melt using the Czochralski method.
In the Czochralski method, conventionally, a desired single crystal of semiconductor is pulled up vertically from a rotating melt of the same semiconductor in a horizontal plane using a seed crystal while the growing single crystal is rotated in an opposite direction to the melt. The melt is held in a crucible and is applied with heat from a heater mounted around the crucible. The crucible containing the melt is mechanically rotated in a horizontal plane in the whole growth process. This is to make the temperature distribution in the melt axisymmetrical to the vertical pull shaft for the crystal (i.e., the growth axis of the crystal). Due to the mechanical rotation of the crucible, the concentration of dopant or dopants introduced into the crystal varies.
Also, the concentration of dopant(s) introduced into the growing crystal varies due to segregation at the interface of the growing crystal and the melt as the growth time increases. Thus, unless the dopant concentration is well controlled, it tends to differ conspicuously from each other between the early and later stages of the crystal growth process. Taking this disadvantage into consideration, both the crystal and the crucible are rotated so as to uniformize the dopant concentration in the crystal thus grown.
With the above-described conventional Czochralski method where the crystal and the crucible are mechanically rotated in the growth process, there is a tendency that the rotation of the growing crystal becomes more difficult with the increasing diameter of the crystal. In particular, this tendency induces a serious problem in the crystal growth of silicon.
Specifically, the crucible made of fused silica is used for growing single crystal of silicon and therefore, oxygen existing in silica tends to dissolve into the growing crystal. For this reason, the concentration of oxygen needs to be well controlled along with the concentration of intended dopant during the growth process. In the above-described conventional method where the crystal and the crucible are mechanically rotated, however, it is difficult to suppress the axial fluctuation of the dopant concentration along the pull shaft in the growing crystal within 1%. Also, to mechanically rotate the large-diameter crucible, a large-scale apparatus or subsystem is necessary. As a result, it has been becoming more difficult to grow a large-diameter single crystal of silicon.
The difficulty in the above-described conventional method can be solved by the technique disclosed in the Japanese Patent No. 2,959,543 issued in October 1999, which was created by the inventors of the present invention, M. Watanabe and M. Eguchi. With the technique disclosed in this patent, a specific magnetic field is applied to a melt of semiconductor and at the same time, electric current is supplied to the melt so as to be perpendicular to the magnetic field. Thus, the radial fluctuation of dopant concentration in a grown crystal is uniformized.
FIG. 1 shows the configuration of the prior-art semiconductor crystal growth apparatus disclosed in the above-identified Japanese Patent No. 2,959,543.
As shown in FIG. 1, the prior-art apparatus comprises a crystal growth furnace 120 with a chamber 109, a coil unit 110 for generating a specific magnetic field which is mounted to surround the furnace 120, and a power supply 104 provided outside the furnace 120. In the chamber 109, a crucible 105 and a heater 108 are mounted. The heater 108 is located to surround the crucible 105. The heater 108 is used to heat a semiconductor raw material in the crucible 105, thereby producing a melt 102 of the semiconductor in the crucible 105. The crucible 105 is used to hold the semiconductor raw material and the melt 102 therein. FIG. 1 shows the state where the melt 102 has been produced with the heater 108 and is held in the crucible 105.
A vertical pull or lift shaft 106, which is made of an electrically conductive material, is provided over the crucible 105. Similar to the ordinary Czochralski method, a seed crystal (not shown) is attached to the bottom end of the shaft 106. The top end of the shaft 106 is supported by a pull or lift mechanism 112. The mechanism 112 serves to pull up or lift vertically the shaft 106 (i.e., a growing single crystal 101 of semiconductor) while rotating the shaft 106 around its axis (i.e., the pull or growth axis).
The coil unit 110 is electrically connected to a power supply (not shown) and is supplied with a specific electric current. Thus, the unit 110 generates a specific magnetic field 111 in the crucible 105.
Electrodes 103 are vertically provided near the crucible 105 so as to be arranged axisymmetrical to the shaft 106. The bottoms of the electrodes 103 are immersed in the melt 102. In FIG. 1, only one of the electrodes 103 is shown for simplification.
One of the two output terminals of the dc power supply 104 is electrically connected in common to the top ends of the electrodes 103 by way of an ammeter 121. The other of the output terminals of the supply 104 is electrically connected to the shaft 106 by way of a resistor 122. A voltmeter 123 is electrically connected in parallel to the resistor 122.
With the prior-art apparatus shown in FIG. 1 having the above-described configuration, in the growth process, the semiconductor raw material is supplied into the crucible 105 and heated with the heater 108, producing the melt 102 of semiconductor in the crucible 105. A bar-shaped single crystal 101 of semiconductor is grown by pulling the seed crystal up from the melt 105 thus produced using the shaft 106. At this time, to prevent the dislocations existing in the seed crystal from propagating to the single crystal 101, a so-called xe2x80x9cneckxe2x80x9d 107 is formed between the seed crystal and the top end of the growing single crystal 101. The neck 107 is a constricted part of the crystal 101 and is formed at the initial stage of the growth process.
During the growth process of the crystal 101, the coil unit 110 is supplied with a specific electric current from the power supply, thereby generating the magnetic field 111 in the chamber 109. The magnetic field 111 thus generated is perpendicular to the interface of the melt 102 and the crystal 101 and axisymmetrical to the shaft 106 in the crucible 105.
Moreover, a specific dc voltage is applied across the electrodes 103 and the pulling shaft 106 by the power supply 104, thereby supplying a specific electric current to the melt 102 existing in the crucible 105. The electric current thus supplied flows through the melt 102, resulting in the Lorentz force applying to the melt 102.
Thus, rotational forces centering on the pulling shaft 106 (i.e., the growth axis) are generated in the melt 102, causing rotation of the melt 102 around the shaft 106 in the crucible 105. As a result, because of stir of the melt 102 by its rotation, the radial fluctuation of the dopant concentration in the grown crystal 101 is uniformized.
Furthermore, the Japanese Patent Nos. 2,950,332 issued in September 1999, 2,885,240 issued in April 1999, and 2,930,081 issued in August 1999 disclose the following techniques relating to the crystal growing apparatus shown in FIG. 1.
In the technique disclosed in the Japanese Patent No. 2,950,332, at least one of the magnetic field applied to the melt of semiconductor and the electric current supplied to the melt is suitably adjusted. Thus, the axial fluctuation of dopant concentration is uniformized.
In the technique disclosed in the Japanese Patent No. 2,885,240, the electrodes, the bottom ends of which are immersed into the melt, are made of the same semiconductor material as the single crystal to be grown. Thus, the introduction of impurity other than the intended dopant into the single crystal is suppressed.
In the technique disclosed in the Japanese Patent No. 2,930,081, the electrodes, which are used to supply the electric current to the melt and the bottom ends of which are immersed into the melt, are respectively inserted into tubes made of the same semiconductor material as the single crystal to be grown. Thus, the symmetry degradation of the temperature distribution in the melt, which is induced by inserting the electrodes into the melt, is prevented from degrading. As a result, the radial distribution of dopant in the single crystal is uniformized.
With the recent crystal growth methods using the Czochralski growth method, generally, as described earlier, xe2x80x9cconstrictionxe2x80x9d of the crystal 101 is performed to prevent the single crystal 101 from containing dislocations. Therefore, the neck 107 is essentially formed between the growing single crystal 101 and the seed crystal. However, it was found that the neck 107 causes the following problem.
With the prior-art apparatus shown in FIG. 1, the electric current supplied by the power supply 104 flows into the melt 101 of semiconductor through the growing crystal 101 and the shaft 106 and therefore, heat generation occurs at the neck 107. This is due to the fact that the neck 107 is higher in electrical resistance than its remaining part. Accordingly, as the pull or lift length of the crystal 101 becomes large and its weight increases, there is an increase danger that the heated neck 107 will break.
For example, when the crystal 101 is single-crystal silicon, it will be 100 kg or greater in weight if it is 20 cm in diameter and 150 cm or more in length. Similarly, the single-crystal silicon crystal 101 is as large in diameter as 30 cm or more in length. In this case, the prior-art apparatus shown in FIG. 1 is unable to pull up the single-crystal silicon crystal 101 as heavy as 100 kg or more having, for example, the above dimensions.
This is applicable to the techniques disclosed in the above-described Japanese Patent Nos. 2,950,332, 2,885,240, and 2,930,081 as well.
Accordingly, an object of the present invention is to provide an apparatus for and a method of growing a single crystal of semiconductor that make it possible to pull up (i.e., grow) a heavy single crystal of semiconductor of 100 kg or greater in weight even if a growing single crystal contains the neck.
Another object of the present invention is to provide an apparatus for and a method of growing a single crystal of semiconductor that prevent the neck of a growing single crystal from generating heat due to an electric current flowing through the neck.
Still another object of the present invention is to provide an apparatus for and a method of growing a single crystal of semiconductor that prevent the neck of a growing single crystal from breaking due to the own weight of the crystal during the growing process.
A further object of the present invention is to provide an apparatus for and a method of growing a single crystal of semiconductor that make it possible to pull up (i.e., grow) a heavy single crystal of semiconductor of 100 kg or greater in weight while the radial and axial dopant concentrations in the single crystal are kept substantially uniform.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, an apparatus for growing a single crystal of semiconductor using the Czochralski method is provided. This apparatus comprises:
(a) a crucible for holding a melt of semiconductor;
(b) a heater for heating the crucible;
(c) a pulling mechanism for pulling up a single crystal of semiconductor from the melt held in the crucible using a seed crystal;
(d) a first power supply having a first terminal and a second terminal;
(e) a first electrode having a first end and a second end;
the first end of the first electrode being electrically connected to the first terminal of the first power supply;
the second end of the first electrode being designed to contact the melt held in the crucible;
(f) a second electrode having a first end and a second end;
the first end of the second electrode being electrically connected to the second terminal of the first power supply;
the second end of the second electrode being designed to contact the melt held in the crucible;
(g) a magnetic field generator for generating a magnetic field in the melt held in the crucible;
wherein in a growth process, a specific voltage is applied across the first ends of the first and second electrodes with the power supply, thereby forming an electrical current path interconnecting the second ends of the first and second electrodes in the melt held in the crucible;
and wherein a magnetic field is generated with the magnetic field generator to intersect with the electrical current path in the melt held in the crucible in the growth process;
and wherein the single crystal is grown to form a neck between the seed crystal and a head end of the single crystal;
With the apparatus according to the first aspect of the present invention, the first and second electrodes are provided in such a way that the first ends of the first and second electrodes are electrically connected to the first power supply and the second ends of the first and second electrodes are contacted with the melt in the crucible.
Also, during the growth process, a specific voltage is applied across the first ends of the first and second electrodes, thereby forming the electrical current path interconnecting the second ends of the first and second electrodes in the melt. The magnetic field is generated with the magnetic field generator to intersect with the electrical current path in the melt.
Accordingly, during the growth process, an electric current flows into the melt through the first electrode and flows out of the melt through the second electrode, and vice versa. This means that no electric current flows through the growing single crystal from the melt. Thus, no heat generation occurs at the neck formed between the seed crystal and the head end of the single crystal even if the single crystal is grown under flow of the melt induced by interaction between the magnetic field and the electric current. This prevents the neck of the growing single crystal from breaking due to the own weight of the single crystal during the growing process.
As a result, with the apparatus according to the first aspect of the invention, a heavy single crystal of semiconductor of 100 kg or greater in weight can be pulled up (i.e., grown) even if the growing single crystal contains the neck. Moreover, the heavy single crystal of semiconductor can be grown while the radial and axial concentrations of dopant in the single crystal are kept substantially uniform.
In a preferred embodiment of the apparatus according to the first aspect of the invention, the second ends of the first and second electrodes are designed to contact with a surface of the melt held in the crucible. The magnetic field generated with the magnetic field generator is approximately perpendicular to the surface of the melt.
In this embodiment, it is preferred that the first and second electrodes are arranged to be axisymmetrical to a pull-up axis of the pulling mechanism.
According to a second aspect of the present invention, a method of growing a single crystal of semiconductor using the Czochralski method is provided. This method comprises the steps of:
(a) providing a melt of semiconductor held in a crucible using a heater;
(b) providing a first power supply having a first terminal and a second terminal;
(c) providing a first electrode having a first end and a second end in such a way that the first end of the first electrode is electrically connected to the first terminal of the first power supply and the second end of the first electrode contacts the melt held in the crucible;
(d) providing a second electrode having a first end and a second end in such a way that the first end of the second electrode is electrically connected to the second terminal of the first power supply and the second end of the second electrode contacts the melt held in the crucible;
(e) applying a specific voltage across the first ends of the first and second electrodes with the power supply, thereby forming an electrical current path interconnecting the second ends of the first and second electrodes in the melt held in the crucible;
(f) generating a magnetic field to intersect with the electrical current path formed in the melt held in the crucible;
(g) pulling up a seed crystal from the melt held in the crucible along a specific growth axis, thereby growing a single crystal of semiconductor from the melt while a neck is formed between the seed crystal and a head end of the growing single crystal.
With the method according to the second aspect of the present invention, because of the same reason as shown in the apparatus according to the first aspect of the invention, the same advantages as those in the apparatus are given.
In a preferred embodiment of the method according to the second aspect of the invention, no electric current is supplied to the single crystal during a whole growth process of the single crystal.
In this embodiment, it is preferred that at least one of an electric current flowing through the electric current path formed in the melt and the magnetic field generated in the melt is adjusted to uniformize a dopant concentration in the single crystal in the growth process of the single crystal.